The present invention relates to tank venting systems and particularly to devices for controlling discharge of fuel vapor from a vehicle fuel tank. More particularly, the present invention relates to tank venting devices for limiting the discharge of fuel vapor from vehicle fuel tanks under high tank pressure conditions.
It has long been appreciated in the design of vehicle fuel systems that it is desirable to provide devices which vent fuel vapor from the vehicle fuel tank at a relatively high flow rate to maintain acceptable levels of fuel vapor pressure in the vehicle fuel tank. Fuel systems engineers have experimented with a wide variety of valve structures to accomplish proper fuel vapor venting. In general, these devices open to allow fuel vapor to vent from the fuel tank over a range of vehicle fuel tank pressures during normal vehicle operation. The devices then close to prevent fuel vapor venting from the fuel tank during, for example, the refueling process. In many of these devices, it is the pressure of the fuel vapor itself which opens the device to allow fuel vapor venting. See, for example, U.S. Pat. Nos. 5,116,257; 5,028,244; and 4,953,583.
Vacuum-actuated tank venting devices are also known. When the engine is running during normal vehicle operation, the resultant vacuum might be used to actuate the venting device to allow fuel vapor venting. See, for example, U.S. patent application Ser. No. 07/660,340, relating to a vacuum-actuated vent assembly.
Regardless of the specific internal structure of the venting device, fuel vapor vented through a venting device typically is directed to a vapor recovery canister or the like. The vapor recovery canister is normally a canister filled with activated carbon or other suitable adsorbent material. Fuel vapor discharged from the fuel tank through the venting device is adsorbed onto the activated carbon, and the exit air stream is vented to the atmosphere. When the engine is operating, the engine may withdraw adsorbed fuel vapor from the activated carbon for subsequent use.
One problem with the use of a vapor recovery canister in conjunction with a tank venting device is that at excessively high fuel vapor flow rates, fuel vapor may tend to overload or otherwise disrupt the operation of the vapor recovery canister. That is, the flow rate of fuel vapor entering the vapor recovery canister may become so high that some amount of fuel vapor may pass through the vapor recovery canister to the atmosphere without becoming adsorbed onto the activated carbon bed and trapped in the canister. In addition, as the engine operates to withdraw fuel vapor from the vapor canister, an overloaded canister may cause the engine to withdraw an excessive amount of fuel vapor, resulting in an undesirably rich fuel/air mixture being introduced into the engine and adversely affecting engine performance.
The problem of overloading the vapor recovery canister with fuel vapor is exacerbated, of course, by the continued efforts to develop venting devices which are configured to vent larger and larger volumes of fuel vapor from the fuel tank to the vapor recovery canister in response to higher and higher pressure levels in the tank. It is thus an object of the present invention to provide an apparatus for controlling the discharge of fuel vapor from a vehicle fuel tank which limits the venting of fuel vapor at pressures higher than a predetermined pressure to avoid overloading the vapor recovery canister with excess fuel vapor.
A fuel vapor venting assembly that is configured to reduce the amount of fuel vapor that is vented from the fuel tank to the vapor recovery canister as the tank pressure rises would be an improvement over conventional tank venting assemblies. What is needed is a tank venting valve that is operable to vent fuel vapor from a tank during normal operating conditions so as to maintain proper tank pressure levels and that is able automatically to step down the flow rate of fuel vapor that is discharged from the tank to the canister as soon as the tank pressure rises to a predetermined level so as to prevent the canister from being overloaded with fuel vapor during high pressure conditions in the tank.
According to the present invention, an apparatus is provided for controlling discharge of fuel vapor from the fuel tank. The apparatus includes a housing defining an interior region, means for defining an outer and an inner flow passageway for fuel vapor in the interior region, and means for selectively blocking flow of fuel vapor through the outer flow passageway. The blocking means is moveable between an unblocking position and a blocking position. The apparatus also includes means for yieldably biasing the blocking means toward the unblocking position.
In preferred embodiments, the apparatus is a multi-stage valve assembly that operates automatically to maximize discharge of fuel vapor from a tank to a canister at low tank pressures and to limit the amount of fuel vapor that is discharged to the canister once the tank pressure rises above a predetermined level. Advantageously, the improved multi-stage valve assembly reduces tank exhaust vapor flow at relatively high tank pressures to prevent canister saturation and possible engine operating problems.
The multi-stage valve assembly includes a vent channel formed to include a small inner flow passageway and a larger annular outer flow passageway surrounding the inner flow passageway. The assembly also includes a valve disk that is movable relative to the vent channel and is formed to include an orifice aligned with the inner flow passageway. Essentially, at low tank pressure, the valve disk is located away from the vent channel to permit fuel vapor in the tank to pass through the valve disk orifice, and then through both of the inner and outer flow passageways into a plenum that is connected to the canister. By using both flow passageways a large volume of fuel vapor can be discharged to the canister at low tank pressure. However, once the tank pressure rises to exceed the predetermined level, the valve disk moves automatically to engage the vent channel and block any further discharge of fuel vapor through the larger annular outer flow passageway. Advantageously, by limiting the amount of fuel vapor discharged from the tank to that amount which is able to pass through the orifice in the valve disk and the small inner flow passageway, the assembly prevents overloading of the canister with fuel vapor.
In the illustrated embodiment, the housing is formed to include an inlet for receiving fuel vapor in the interior region from the fuel tank and an outlet for discharging fuel vapor from the interior region to a vapor recovery device. The defining means cooperates with the housing to define an annular space to provide the outer flow passageway. The defining means is also formed to include the inner flow passageway. The blocking means is formed to include a high flow orifice allowing passage of fuel vapor therethrough. The blocking means is movable in the interior region in response to fuel vapor pressure. When positioned in the unblocking position, the blocking means is out of engagement with the defining means, allowing the fuel vapor passing through the high flow orifice to enter and pass through both of the inner and outer flow passageways. When positioned in the blocking position, the blocking means is in engagement with the defining means, allowing fuel vapor passing through the orifice to enter and pass through only the inner flow passageway to reduce the flow of fuel vapor to the vapor recovery device.
Advantageously, then, the present invention steps down the flow of fuel vapor to the vapor recovery device when the fuel tank pressure exceeds a predetermined level. This reduces the potential for overloading the vapor recovery device with fuel vapor and for providing an excessively rich fuel/air mixture to the engine.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.